So-called sandwich structure injection molding is one of injection molding techniques. In this injection molding, two types of resin materials are injected into a mold cavity to mold a multilayered structure containing a resin molded portion for forming a core layer and a resin molded portion for forming a skin layer.
Many prior art references pertaining to this sandwich molding are available, and they are classified into International Patent Classification IPC B29C 45/16. Examples are Japanese Patent No. 3017052 and Japanese Patent Laid-Open No. 8-224754.
The present invention also relates to a technique of reusing molded plastic materials and resin materials. Examples of prior art references related to the reuse of resin materials are Japanese Patent Laid-Open Nos. 5-301222, 5-147036, and 10-202694.
Japanese Patent No. 3031357 describes an example in which a recycled material is used as a core material of a sandwich molded product and a recycled material is also used as a surface material.
Recently, industrial circles are variously influenced to protect the global environment and effectively use natural resources. Also, attempts to reuse plastic materials and resin materials as petroleum resources have been demanded to be put into practical use.
To meet this demand, development of resin molded product recycling techniques is being extensively made.
In particular, relatively large amounts of a number of large-sized thermoplastic materials are used in various types of electronic apparatuses, e.g., image forming apparatuses such as copying machines and printers, office machines and information communication apparatuses such as facsimile apparatuses and computers, and household electric appliances such as air conditioners, video decks, and audio components.
Thermoplastic materials removed from used products of these apparatuses are inferior in diverse physical properties such as mechanical strength, fire retardance, and external color.
Also, various foreign substances are sticking to the surface to contaminate it.
Accordingly, as a method of recycling these materials, a method has been proposed by which molded products are pulverized, cleaned, and repelletized, and an unused resin material (virgin pellets) and the used resin material are mixed at a predetermined ratio to recover the physical properties.
In this method, however, a deteriorated used resin material is mixed in an unused resin material. Therefore, a manufactured recycled material is inferior to a molded product of a 100% virgin pellet resin material in mechanical strength, fire retardance, and external color.
To solve the environmental problems by effectively using the aforementioned resources and reducing the amount of industrial waste as much as possible, multilayered structure molding, so-called sandwich molding, can be used as one means for using a used resin material as the material of a resin molded product.
That is, this multilayered molded product is obtained by injecting a resin material for forming a core layer and a resin material for forming a skin layer into a mold, thereby forming a resin molded product having a multilayered structure in which the core layer is covered with the skin layer.
The method uses the used resin material described above as the core layer resin material of this multilayered structure.
To increase the resin material recycling ratio by reusing a used resin material as a raw material of the above-mentioned multilayered structure molding, i.e., so-called sandwich molding, the filling ratio of a core layer resin portion as a recycling material is important.
When the filling amount of a core layer structural portion is increased and the skin layer resin material is injected around the core layer structural portion, the core layer resin sometimes overflows from a portion covered with the skin layer portion.